The invention relates to a process and a device for manufacturing cement clinker from raw material pellets containing the constituents necessary for manufacturing the clinker.
It is known that the clinker used in the composition of cement is obtained by calcining an intimate mixture of limestone and clay in a cement manufacturing kiln.
Apart from the traditional wet or dry processes in which the starting materials are intimately mixed to form a starting material in the form of a paste or a powder which is then fired, it has more recently been proposed to use as the starting material small balls or pellets consisting of agglomerated powders having homogeneous shapes and dimensions.
The conventional wet processes consume substantial quantities of energy, of the order of 1500 calories per kg of clinker.
Conventional dry processes which carry out the clinkering process in a rotary kiln consume considerably less energy, of the order of 850 calories per kg of the clinker. However, the theoretical lower limit of energy consumed for the production of the clinker is in the region of 430 calories per kg. It will thus be seen that the actual yield of cement manufacturing plants is only slightly greater than 50%.
Actual processes using the dry method use rotary kilns of very great length producing a clinker of extremely variable granulometry which leaves the kiln at a very high temperature.
A certain amount of the heat from the clinker leaving the rotary kiln is recovered by making use of coolers arranged at the outlet of such kilns. These coolers, do however, have a very poor yield as a result of the extremely variable granulometry of the clinker produced and of the difficulty of regulating, at all points, the air flow to the clinker to be cooled. Moreover, pre-heating of the raw materials is carried out before they enter the kiln, using recovery of the heat from the gases leaving the clinkering kiln. However, as the material entering the kiln is in the form of a powder, the heat exchange process is carried out using counter-flow in the pipes in which the hot gases carrying the material introduced along with them are circulating. It is consequently necessary to use a heat exchanger with several cyclone stages, each cyclone receiving the gases originating from the previous stage charged with the material recovered from the next stage and separating the material for re-introducing it into the feed pipe of the previous stage.
Consequently, although the powder is carried along in counter-flow by the gases, the use of several cyclone stages makes it possible to establish, right up to the point of entry into the rotary kiln, an overall circulation of the material in counter-flow to the gases, and in so doing, to partly improve the exchange of heat. However, this makes it necessary, in those installations producing large tonnages, to provide preheating towers of very considerable height, of the order of 50 m, and the temperature of the gas leaving the preheating plant is still very high, of the order of 300.degree. C.
The overall thermal efficiency of cement manufacturing installations consequently remains at a relatively low level, despite progress which has been achieved by using the dry method in which use is made of techniques such as precalcining the solid matter before it is fed into the cement making kiln. In actual practice, the precalcining process which has the purpose of decarbonating the matter before it is introduced into the rotary kiln facilitates the job to be carried out by the latter, but does not substantially change the overall thermal yield of the process. It does, however, make it possible to use a shorter rotary kiln.
The introduction of solid products in powder form into the kiln, and the recovery at the outlet from the kiln of solid products of variable granulometry, complicates the thermal exchange carried out in the cooler and has the effect of producing pre-heating temperatures for the air which are relatively low. Additionally, it prevents use, in the plant of, the total amount of air pre-heated in this way.
Proposals have been made to carry out the shaping of the raw materials by forming them into pellets before they are introduced into the calcining plant which can take the form of a kiln with a vertical axis comparable to a tunnel kiln, where the products are able to circulate by gravity by forming a movable bed or in suspension in a gas. A heat exchanger operating on counter-flow principles has an excellent yield if care is taken to adjust accurately the flow rate of the materials and the fluids, taking into account their respective specific heats, and if such fluids are suitably distributed within the mass of the materials.
For this purpose, it is advantageous to divide the apparatus into three super-posed zones comprising an upper exchanger, a clinkering zone using a fluidized bed and a lower exchanger. Such a device is, for example, described in British Pat. No. 1,046,617.
The raw materials, in the form of pellets, are introduced into the upper portion of the device and circulate firstly in a pre-heating zone before being introduced into the central calcining zone where a burner makes it possible to raise the temperature of the product to the temperature needed for clinkering. The hot gases which are recovered from this zone are used for pre-heating the raw materials introduced into the installation, and the calcined materials leaving in calcining zone at high temperature pass to a cooling zone, where they come into contact with a flow of air which is circulating in the installation in counterflow with respect to the circulation of the solid matter.
In such a device, all the heat introduced is produced in the clinkering zone which makes use of reactions which include a large proportion of exothermic reactions. Moreover, although the solid matter has been pre-heated by using the hot gases from the clinkering zone, a considerable amount of heat energy must additionally be supplied in order to reach the clinkering temperature, which is in the region of 1450.degree.. It is in fact necessary to obtain, within these zones, a supply of heat which is all the greater because decarbonation of the raw materials at a temperature of 900.degree. requires more than 400 cal/kg of clinker (amounting to 90% of the energy total, within this zone). The pellets which form a fluidized bed in the clinkering plant must consequently be in contact with hot combustion gases, where large temperature differences between the fluid and the solid matter encourage the occurrence of caking and necessitate a relatively long clinkering time.
The invention has the aim of overcoming this disadvantage by providing, in an installation of this type, a clear separation between the zone where decarbonation takes place and the clinkering zone. Operating in this way, each reaction is carried out at the appropriate temperature, under homogeneous conditions, and the amount of heat which is strictly necessary is introduced into each zone, making it possible to provide economies in the overall fuel consumption, to improve the quality of the clinker obtained and to reduce the risk of caking.